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<?php
/**
 * CCacheDependency class file.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @link http://www.yiiframework.com/
 * @copyright Copyright &copy; 2008-2011 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

/**
 * CCacheDependency is the base class for cache dependency classes.
 *
 * CCacheDependency implements the {@link ICacheDependency} interface.
 * Child classes should override its {@link generateDependentData} for
 * actual dependency checking.
 *
 * @property boolean $hasChanged Whether the dependency has changed.
 * @property mixed $dependentData The data used to determine if dependency has been changed.
 * This data is available after {@link evaluateDependency} is called.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @version $Id$
 * @package system.caching.dependencies
 * @since 1.0
 */
class CCacheDependency extends CComponent implements ICacheDependency
{
	private $_data;

	/**
	 * Evaluates the dependency by generating and saving the data related with dependency.
	 * This method is invoked by cache before writing data into it.
	 */
	public function evaluateDependency()
	{
		$this->_data=$this->generateDependentData();
	}

	/**
	 * @return boolean whether the dependency has changed.
	 */
	public function getHasChanged()
	{
		return $this->generateDependentData()!=$this->_data;
	}

	/**
	 * @return mixed the data used to determine if dependency has been changed.
	 * This data is available after {@link evaluateDependency} is called.
	 */
	public function getDependentData()
	{
		return $this->_data;
	}

	/**
	 * Generates the data needed to determine if dependency has been changed.
	 * Derived classes should override this method to generate actual dependent data.
	 * @return mixed the data needed to determine if dependency has been changed.
	 */
	protected function generateDependentData()
	{
		return null;
	}
}